In order to meet the thrust requirements of military applications, military turbo-fan engines employ after-burning. During after-burning, fuel is added and ignited in the exhaust of a turbo-fan engine to create additional thrust. A consequence of after-burning is that when the exhaust burns, it becomes less dense and requires opening the turbo-fan engine exhaust nozzle to maintain air flow rate at a level that is acceptable for proper engine operation. An insufficient exhaust nozzle area limits this air flow rate to create back-pressure on the fan. This back-pressure can cause the engine to stall. One approach to preventing engine stall during after-burning is to use a adjustable exhaust area nozzle that permits control of the cross-sectional nozzle exhaust flow area during after-burning. This allows the exhaust that the engine burns to escape more easily from the turbo-fan engine. As a result, the back pressure subsides. The opening and closing of the adjustable exhaust area nozzle has the appearance of an eye iris as the eye reacts to changes in light intensity.
Using an engine with an adjustable exhaust nozzle, however, has many inherent disadvantages which penalize aircraft performance. Some of the disadvantages relate to the mechanical aspects of the engine nozzle. The adjustable exhaust nozzle has greater mechanical complexity, greater weight, and greater fabrication and operation costs does than an engine not having such a nozzle. At the same time, such nozzles offer poor reliability and maintainability, integrate poorly into the aerodynamic contours of the air frame and have edges and gaps that increase the aircraft's radar signature. In addition, the moveable surfaces of such nozzles are difficult to cool. This makes reducing the infrared signature of the aircraft either impossible or impractical.
The radar and infrared signature disadvantages of the adjustable exhaust area nozzle prevent using engines with after-burning capability in some military applications, such as those aircraft designed to have low radar and thermal signatures. Currently available military aircraft that incorporate into their airframe some form of stealth protection all must have fixed geometry nozzles. Because these nozzles must be fixed, there is no after-burning capability for these aircraft. The lack of after-burning capability prevents these aircraft from using the additional thrust that after-burning would make available. As a result, incorporating the additional stealth characteristics into the airframe comes at the cost of additional vulnerability to the crew from their not having the additional thrust that after-burning provides.
One attempt to eliminate the inherent disadvantages of an adjustable exhaust area nozzle is to use a fixed aperture nozzle at the after-burning engine exhaust while varying the internal flow area of the exhaust nozzle. An inherent problem with this approach is that maintaining the exit aperture fixed produces a non-optimum area ratio in the nozzle. The non-optimum area ratio, therefore, causes a loss of engine thrust.
A need exists, therefore, for a turbo-fan engine with after-burning capabilities that overcomes back-pressure problems of existing fixed nozzle configurations, and that avoids the limitations of known adjustable exhaust area nozzles.
A need exists for an improved after-burning turbo-fan engine that is mechanically simple, lightweight, and inexpensive relative to the adjustable exhaust area nozzle configurations.
There is a further need for a turbo-fan engine having a fixed geometry exhaust nozzle that possesses high reliability and maintainability and that does not adversely affect integration of the nozzle into the airframe or produce undesirable radar and infrared signatures from the nozzle.